Every living organism exhibits various responses against the external environment in order to maintain the homeostasis of life. The synthesis of melanin also relates to this phenomenon.
In general, there are various reasons for the darkening of skin color, the main reason being ultraviolet rays. When skin is exposed to ultraviolet rays, melanin is synthesized in melanocytes, which is a type of skin cell, and released to darken skin color. In the process of melanin synthesis in melanocytes, tyrosinase reacts on tyrosine, which is a substrate for tyrosinase, in the cell to yield Dopaquinone and it goes through sequential oxidation to give Dopachrome and then 5,6-dihydrozyindole (DHI) and 5,6-dihydroxyindol-2-carboxylic acid (DHICA), the monomers of melanin, which are then polymerized with each other to provide a copoylymeric black pigment, melanin. Thus, an understanding of the mechanism of melanin synthesis is very important to the development of whitening agents or cosmetic products having whitening effect. In other words, the interpretation of the melanin-control mechanism may provide a method of preventing color deposition by melanin. In order to prevent darkening of skin color (for example, liver spots, freckles, or the like), it is most simple and general to inhibit a step of the process of generating melanin, and to reduce the production of melanin. Such inhibition can be performed by inhibiting the activity of tyrosinase.
At present, research activity to develop an agent with tyrosinase-inhibiting activity is intensive. Representatives of inhibiting agents include kojic acid, arbutin, hydroquinone, Vitamin C, Cortex mori, and licorice root extract.
Among these, kojic acid forms a chelate with a copper ion at the active site of tyrosinase to inhibit the enzyme activity. Though it has high activity, it is not appropriate for use in cosmetics because of stability problems in the process of blending in cosmetic products.
Ascorbic acid (vitamin C) and its derivatives have relatively low activity in inhibition of tyrosinase.
Hydroquinone is not desirable in that it has irreversible whitening activity and irritates the skin strongly, so that its use as a cosmetic material has been restricted in the past. Furthermore, hydroquinone has recently been recognized as a carcinogen, so that its use in cosmetic products is now prohibited.
Many plant extracts have demonstrated substantial inhibition effect on tyrosinase activity, when used in high concentrations. When used in relatively low concentrations, tyrosinase inhibition activity is hardly seen to occur.
In view of the above, the present inventors have performed intensive studies for finding a superior and excellent whitening agent which avoids the problems of conventional whitening agents. As a result of searching for an effective material having whitening activity among the natural plants of which the safety has been already proved as they have been used in herb remedies or folk remedies for a long time, it has been found that the extracts of Ramulus mori, young twigs of plants belonging to Morus genus, show excellent inhibiting activity on tyrosinase. The results of these studies have been filed as an invention with the Korea Industrial Property Office [Korean Patent Application No. 97-4913]. However, the actual compound having inhibition effect on tyrosinase activity among the materials contained in Ramulus mori has not been described in the above patent application. Thus, the development of an excellent and superior inhibiting agent for melanin synthesis by isolating and purifying the tyrosinase-inhibiting components of Ramulus mori is still urgently required.